The invention relates to apparatus for separating dust from a dusty gas. More specifically, the invention relates to gas filters in which the dusty gas flows horizontally into a treatment chamber, and the filtering material flows vertically into the treatment chamber.
For an understanding of problems associated with prior art devices a prior art gas filtering apparatus is illustrated in FIG. 12. In the prior art apparatus 31, a housing 32 houses two screens 33, 33a defining a treatment chamber 64 into which particulate filter medium 34 and dust-laden gas from gas inlet 42 are fed and contacted. As the dust-laden filter medium particles 34 descend into chute 36, they are directed to rotary drum discharge apparatus 37 placed at the bottom of chute 36.
As the rotary drum discharge apparatus 37 rotates, dust-laden filter medium 34 is discharged onto vibrating sieve 38 on which dust 39 is shaken off of the dust-laden filter medium 34; and, cleaned filter medium 40 is recycled back into the gas treatment chamber 64.
The screens 33, 33a which define the gas treatment chamber 64 are designed to prevent particles of the filter medium 34 from flowing out of the gas treatment chamber 64 but allow the dust-laden gas that is treated to flow into and out of the gas treatment chamber 64.
Conventional dust filters as illustrated in FIG. 12 have several disadvantages. One disadvantage relates to elimination efficiency, and another disadvantage relates to treatment performance. Elimination efficiency is defined as the ratio of the difference between the dust concentration at the inlet of the gas treating chamber and the dust concentration at the outlet of the gas treating chamber divided by the dust concentration at the inlet (Di-Do)/Di.
The treatment performance is defined as the volume of the gas treated per unit of time (e.g. Nm.sup.3 /Sec. where N denotes the gas volume under standard temperature and pressure conditions and m.sup.3 is cubic meters).
With the conventional dust filter, as the descending speed of the filter medium particles becomes slower, the elimination efficiency of the dust increases. This results in an increase in the pressure drop caused by the dust-laden gas because a larger amount of dust adheres to the filter medium particles and reduces the open spaces between the particles. Increased pressure drop across the treating chamber causes a decrease in the throughput and results in decreased treatment performance. If the pressure drop is great, a large blower with sufficient power to overcome the pressure drop and to increase treatment performance is required. A large blower consumes relatively large amounts of electric power and raises costs.
On the other hand, if the rate of descent of the filter medium particles in the gas treatment chamber is high, the amount of dust which adheres to the filter medium particles is small, and the pressure drop of the gas is also small. This condition would lessen the energy required to operate the blower, and treatment performance of the gas would increase. However, because, in this instance the spacing between the filter medium particles is relatively large, the dust can easily pass through the wide spaces, and the filter medium particles are unable to adequately separate dust from the gas thereby reducing elimination efficiency.
Thus it is seen that the two important parameters, treatment performance and elimination efficiency, are inversely related in a conventional dust filter apparatus. This is an undesirable situation because it is desirable that both parameters be maximized.
Additional problems are present with conventional dust filter apparatus. It has been observed that the amount of dust adhering to filter medium particles near the gas inlet differs significantly from the amount of dust particles adhering to the filter medium particles near the gas outlet. Thus, elimination efficiency depends upon localized conditions at both inlet and outlet regions. Not only does more dust adhere to the filter medium near the gas inlet but the pressure loss of the gas is also larger at the inlet side of the gas treatment region. In other words, the filter medium at the outlet side of the gas treatment chamber contributes very little to the elimination efficiency of the filter medium. Thus, there are important local effects in the filter medium in a horizontal direction along gas passage from into and out of the gas treatment chamber.
In certain types of conventional prior art dust separators, countercurrent methods are employed. For example, in a water jet gas washing device, highest efficiency is obtained when the direction of gas flow is anti-parallel with the water jet flow. Efficiency is greatest in this arrangement because of the increased probability of collision between dust and water droplets. However, countercurrent separation methods are not known to be in use with dry, dynamic gas filters depicted in FIG. 12.
Accordingly, it is a primary object of the present invention to provide an apparatus for treating dust-laden gas in which both the elimination efficiency and the treatment performance may both be optimized.
Another advantage of the present invetion is the provision of a gas treating apparatus in which local effects in a horizontal direction along gas passage are significantly reduced.